The Xena: Warrior Princess TCG has been out of print for several years now and Wizards of the Coast has no plans to introduce any new cards. This makes it somewhat difficult to pick up starter decks, booster packs etc. Below is a list of places that you can still find cards, many times at greatly reduced prices!

Ebay
I have had great luck finding entire boxes of both booster packs and starter decks. Also, you can find people selling specific cards, collections, etc.

Xena Fansites, Message Boards, Yahoo Groups
Check out the online community of "Xenites" and see what they have to offer.

Local Comic/Collectable Stores
I was lucky enough to find some shops around me that still had some booster packs left on the shelf, never hurts to ask if you don't see any either.